1.
absentminded, distracted, inattentive
A mental state where one's attention is elsewhere and not focused on the present situation or conversation.
Etymology
Literally 'the upper sky' - suggesting that one's mind has floated up and away, not grounded in the present moment.
Common patterns
The expression is often used as a na-adjective: 上の空な態度 (a distracted attitude).
Related Words
Compare:
心ここにあらず (absentminded)